Method to control the active shock absorbers of a road vehicle featuring the lowering of the centre of gravity

ABSTRACT

Method to control active shock absorbers of a road vehicle. Each active shock absorber is part of a suspension connecting a frame to a hub of a wheel and is provided with an actuator. The control method comprises the steps of: determining a longitudinal acceleration and a transverse acceleration of the road vehicle; establishing a desired lowering of a centre of gravity of the road vehicle depending on the longitudinal acceleration and on the transverse acceleration; and controlling the actuator of each active shock absorber so as to obtain the desired lowering of the centre of gravity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Patent Application claims priority from Italian Patent ApplicationNo. 102021000015170 filed on Jun. 10, 2021, the entire disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a method to control the active shock absorbersof a road vehicle.

BACKGROUND ART

The movement of passive shock absorbers is entirely determined by thestresses transmitted by the road surface and, therefore, passive shockabsorbers are “at the mercy” of the road surface. For a few years now,active shock absorbers have been offered, which are capable of makingautonomous movements (namely, completely independent of the stressestransmitted by the road surface), which are added to the movementscaused by the stresses transmitted by the road surface; the aim of theautonomous movements made by an active shock absorber is that ofreacting to the stresses transmitted by the road surface so as tomaximize the dynamic performance of the road vehicle or improve thedriving comfort of the road vehicle (the same road vehicle can have itsactive shock absorbers pursue different targets depending on the type ofdriving chosen by the driver).

An active shock absorber is provided with an electric or hydraulicactuator of its own, which can be controlled so as to generate anautonomous movement (namely, completely independent of the stressestransmitted by the road surface); for example, by controlling theactuator of an active shock absorber, the frame of the road vehicle canbe lowered or lifted in an independent manner on each wheel (even whenthe vehicle is still).

Patent application DE102005053222A1 describes the control of the activesuspensions of a road vehicle, wherein, while driving along a bend, thebody of the road vehicle is actively lowered on both axles and on theinner side of a bend; the kinematic of the active suspensions isdesigned so that, while driving along a bend, the wheels of all axlesare subjected to a variation of camber towards greater negative camberangles.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide a method to control the activeshock absorbers of a road vehicle, which maximizes performances whiledriving in conditions close to the grip limit.

According to the invention, there is provided a method to control theactive shock absorbers of a road vehicle according to the appendedclaims.

The appended claims describe preferred embodiments of the invention andform an integral part of the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings showing a non-limiting embodiment thereof, wherein:

FIG. 1 is a schematic plan view of a road vehicle provided with fouractive shock absorbers, which are controlled according to the invention;

FIG. 2 is a schematic view of a suspension of the road vehicle of FIG. 1;

FIG. 3 is a schematic view of the road vehicle of FIG. 1 while drivingalong a bend, highlighting the trajectory, the driving speed, thesteering angle and the attitude angle;

FIG. 4 is a control diagram implemented in a control unit of the roadvehicle of FIG. 1 ;

FIG. 5 is a diagram showing the variation of a desired lowering of thecentre of gravity of the road vehicle of FIG. 1 as longitudinalacceleration changes;

FIG. 6 is a diagram showing the variation of a desired lowering of thecentre of gravity of the road vehicle of FIG. 1 as transverseacceleration changes;

FIG. 7 is a diagram showing the variation of a desired roll angle of theroad vehicle of FIG. 1 as transverse acceleration changes; and

FIG. 8 is a diagram showing the variation of a desired pitch angle ofthe road vehicle of FIG. 1 as longitudinal acceleration changes.

PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1 , reference number 1 indicates, as a whole, a road vehicleprovided with two front wheels 2 and with two rear wheels 2.

The road vehicle 1 is provided with a powertrain system (known and notshown herein), which can comprise an internal combustion engine and/orone or more electric motors.

A hub 3 (schematically shown in FIG. 2 ) of each wheel 2 is connected toa frame 4 of the road vehicle 1 by means of a suspension 5 (partiallyshown in FIG. 1 ), which is provided with an (electronically controlled)active shock absorber 6, which is capable of making autonomous movements(namely, completely independent of the stresses transmitted by the roadsurface), which are added to the movements caused by the stressestransmitted by the road surface.

According to FIG. 2 , each active shock absorber 6 comprises an element7, which defines an end of the active shock absorber 6, and an element8, which defines the other end of the active shock absorber 6 and ismounted so as to slide relative to the element 7 in order to be able tolinearly translate relative to the element 7. Each active shock absorber6 comprises a spring 9, which is connected between the two elements 7and 8 and is compressed or expanded when the two elements 7 and 8linearly translate relative to one another. Finally, each active shockabsorber 6 comprises an electric actuator 10, which is configured tohave the active shock absorber 6 make autonomous movements (namely,completely independent of the stresses transmitted by the road surface)between the elements 7 and 8, namely is capable of generating a force F,which is applied between the elements 7 and 8. By way of example, theactive shock absorbers 6 could be of the type described in patentapplications US2008190104A1 and W02014145215A2. Each active shockabsorber 6 comprises a position sensor 11 (for example, apotentiometer), which provides the current relative positionp_(1 . . . 4) of the two elements 7 and 8, namely the exact measure ofhow much the element 8 is translated relative to the element 7.

The road vehicle 1 comprises an electronic control unit (“ECU”) 12,which, among other things, controls the actuators 10 of the active shockabsorbers 6 in the ways described below; from a physical point of view,the control unit 12 can consist of one single device or of severaldevices, which are separate from one another and communicate through theCAN network of the road vehicle 1.

According to FIG. 1 , the road vehicle 1 comprises a longitudinalaccelerometer 13 and a transverse accelerometer 14, which are mounted onthe frame 4, namely are rigidly fixed to the frame 4 in order to movewith the frame in an integral manner 4, and are configured to measure alongitudinal acceleration a_(x) and a transverse acceleration a _(y) ofthe frame 4 (namely, of the road vehicle 1), respectively. According toa possible embodiment, the two accelerometers 13 and 14 could beintegrated in one single sensor (for example, a triple-axisaccelerometer), which provides both the longitudinal acceleration a_(x)and the transverse acceleration a_(y). The control unit 12 is connected(directly or indirectly through a BUS network of the road vehicle 1) tothe accelerometers 13 and 14 in order to cyclically read the currentvalues of the longitudinal acceleration a_(x) and of the transverseacceleration a_(y).

The control unit 12 is capable of cyclically reading the current valueof a steering angle a (shown in FIG. 3 ) of the front wheels 2(typically, through the BUS network of the road vehicle 1).

According to FIG. 4 , the control unit 12 implements an estimating block15, which determines, in a known manner, the actual attitude angle β ofthe road vehicle 1 (namely, the angle comprised between the longitudinalaxis x of the road vehicle 1 and the direction of the driving speed V ofthe road vehicle 1 in the centre B of gravity). By way of example, theestimating block 15 of the control unit 12 estimates the trajectory Tfollowed by the road vehicle 1 using the measures provided in real timeby a triple-axis gyroscope and by a GPS tracking unit; in particular,the trajectory T is determined by integrating twice in time theaccelerations measured by the triple-axis gyroscope and the measuresprovided by the GPS tracking unit are used to cyclically cancel theposition errors occurring during the integration process. Furthermore,the estimates block 15 of the control unit 12 estimates the drivingspeed V of the road vehicle 1 in the centre B of gravity using themeasures provided in real time by the triple-axis gyroscope; inparticular, the speed V of the road vehicle 1 in the centre B of gravityis determined by integrating once in time the accelerations measured bythe triple-axis gyroscope (making sure that the driving speed V of theroad vehicle 1 in the centre of gravity B actually is tangent to thetrajectory T followed by the road vehicle 1, otherwise, in case of asignificant deviation, at least one further iteration of the calculationis carried out making corrections to the parameters used).

The control unit 12 implements an estimating block 16, which determinesin a known manner a yaw angle ψ (namely, the angle of oscillation of theroad vehicle 1 around a vertical axis going through the centre B ofgravity) and, as a consequence, a yaw speed Vψ namely the variation ofthe yaw angle ψ over time (the yaw speed Vψ can be determined byderiving the yaw angle ψ in time). According to a possible embodiment,the yaw speed Vψ can be measured and provided by the same sensorintegrating the two accelerometers 13 and 14; namely, the integratedsensor also provides, besides the longitudinal acceleration a_(x) andthe transverse acceleration a_(y), the yaw speed Vψ; the yaw angle ψ canbe determined by integrating the yaw speed Vψ in time.

According to FIG. 4 , the control unit 12 implements a calculation block17, which receives as an input: the positions p_(1 . . . 4) of the fouractive shock absorbers 6 provided by the position sensors 11, thelongitudinal acceleration a_(x) provided by the longitudinalaccelerometer 13, the transverse acceleration a_(y) provided by thetransverse accelerometer 14, the attitude angle β provided by theestimating block 15 and the yaw speed Vψ provided by the estimatingblock 16. The calculation block 17 determines, in ways that will bedescribed more in detail below, a desired lowering h_(b−TGT) of thecentre B of gravity, namely a value (generally expressed in mm), whichindicates the extent to which the centre B of gravity has to be loweredrelative to a standard position assumed by the centre B of gravity inthe absence of external interventions (for example, when the roadvehicle 1 is still). Furthermore, the calculation block 17 determines,in ways that will be described more in detail below, a desired rollangle φ_(TGT) and a desired pitch angle θ_(TGT).

The object of the desired lowering h_(b−TGT) of the centre

B of gravity is that of lowering the road vehicle 1 in dynamicconditions so as to obtain a better performance through a decrease inthe transfer of absolute load, with a final effect of increase in theoverall grip of the tyres of the wheels 2 (namely, each wheel 2 isbasically stressed in the same way, instead of having more stressedwheels 2 and less stressed wheels 2); the decrease in the transfer ofload takes place both in case of lateral slip and in case oflongitudinal slip, with a consequent improvement of performances interms of maximum transverse acceleration a_(y) and maximum longitudinalacceleration a_(x). Hence, the desired lowering h_(b−TGT) of the centreB of gravity also positively affects the acceleration and brakingphases, since the decrease in the transfer of longitudinal load allows agreater force to be transmitted to the ground and, consequently, leadsto a greater maximum longitudinal acceleration a_(x).

The object of the control of the desired roll angle φ_(TGT) is that ofreducing dynamic and static roll; it is possible to define both agradient of static roll and a desired dynamic behaviour as the inputfrequencies change.

The objects of the control of the desired pitch angle θ_(TGT) are thoseof: reducing static and dynamic pitch, changing the attitude of the roadvehicle 1 in order to optimize aerodynamic work points and dampingtransients in those braking actions in which the antilock braking system(ABS) of the wheels 2 intervenes. The reduction of the pitch and theslowing down of the dynamic of the pitch lead to a quicker settling ofvertical and longitudinal forces and improve aerodynamic balance (thusreducing the transfer of longitudinal load); these different effectslead to an improvement of stopping spaces.

According to FIG. 4 , the control unit 12 implements a calculation block18, which receives as an input from the calculation block 17 the desiredlowering h_(b−TGT) of the centre B of gravity, the desired roll angleφ_(PTGT) and the desired pitch angle θ_(TGT) and determines, for eachactive shock absorber 6, a desired force F_(1 . . . 4−TGT) (namely, aforce target), which must be generated by the active shock absorber 6and, hence, is expressed in Newton. Namely, the electric actuator 10 ofeach active shock absorber 6 is controlled so as to pursue thecorresponding desired force F_(1 . . . 4−TGT) i.e. cause thecorresponding desired force F_(1 . . . 4−TGT) to be generated.

According to a preferred embodiment, the calculation block 18 uses amathematical model of the road vehicle 1, which, depending on theinstant-by-instant value of the input variables (desired loweringh_(b−TGT) of the centre B of gravity, desired roll angle φ_(TGT) anddesired pitch angle θ_(TGT)) provides the instant-by-instant value ofthe output variables (desired forces F_(1 . . . 4−TGT) to be generatedby the active shock absorbers 6).

Preferably, the control block 17 implemented in the control unit 12recalculates (updates) the desired lowering h_(b−TGT) of the centre B ofgravity, the desired roll angle φ_(TGT) and the desired pitch angleθ_(TGT) with a relatively low frequency generally ranging from 1 to 4Hz; obviously, the control block 18 implemented in the control unit 12recalculates (updates) the desired forces F_(1 . . . 4−TGT) with thesame update frequency as the control block 17.

The control block 17 implemented in the control unit 12 establishes adesired lowering h_(b−TGT) of the centre B of gravity of the roadvehicle 1 depending on the longitudinal acceleration a_(x) and on thetransverse acceleration a_(y) (as mentioned above, the actuator 10 ofeach shock absorber 6 is controlled so as to obtain the desired loweringh_(b−TGT) of the centre B of gravity through the action of the controlblock 18). In particular, the control block 17 establishes a firstcontribution depending on the sole longitudinal acceleration a_(x)(namely, without taking into account the transverse acceleration a_(y)),establishes a second contribution depending on the sole transverseacceleration a_(y) (namely, without taking into account the longitudinalacceleration a_(x)) and then determines the desired lowering h_(b−TGT)of the centre B of gravity as the greater one, in absolute value,between the first contribution and the second contribution (namely, itchooses the contribution with the higher absolute value). In otherwords, the desired lowering h_(b−TGT) of the centre B of gravity isequal to the contribution that has the greater absolute value.

According to FIG. 5 , the control block 17 establishes the desiredlowering h_(b−TGT) of the centre B of gravity (namely, the firstcontribution of the desired lowering h_(b−TGT) of the centre B ofgravity) depending on the longitudinal acceleration a_(x) by means of alinear law L1 (shown in FIG. 5 ). The linear law L1 is symmetrical, in amirror-like manner, for positive and negative longitudinal accelerationsa_(x) (symmetrical in a mirror-like manner, as the desired loweringh_(b−TGT) of the centre B of gravity always is negative regardless ofthe sign of the longitudinal acceleration a_(x)) and entailsproportionally increasing the desired lowering h_(b−TGT) of the centre Bof gravity as the absolute value of the longitudinal acceleration a_(x)increases. In particular, the linear law L1 entails a zero desiredlowering h_(b−TGT) of the centre B of gravity, when the absolute valueof the longitudinal acceleration a_(x) is smaller than a threshold valueTH1, it entails a constant desired lowering h_(b−TGT) of the centre B ofgravity equal to a maximum value VMAX1, when the absolute value of thelongitudinal acceleration a_(x) is greater than a threshold value TH2,and it entails a linear variation of the desired lowering h_(b−TGT) ofthe centre B of gravity from when the absolute value of the longitudinalacceleration a_(x) is equal to the first threshold value TH1 to themaximum value VMAX1 when the absolute value of the longitudinalacceleration a_(x) is equal to the second threshold value TH2.

In the embodiment shown in FIG. 5 , the maximum value VMAX1 is equal (inabsolute value) to 20 mm (in FIG. 5 , the value VMAX1 is negative toindicate that it is a lowering), the threshold value TH1 is equal) inabsolute value) to 0.25 g and the threshold value TH2 is equal (inabsolute value) to 1 g; letter “g” indicates the average gravitationalacceleration measured on earth, which conventionally amounts to 9.80665m/s².

According to FIG. 6 , the control block 17 establishes the desiredlowering h_(b−TGT) of the centre B of gravity (namely, the secondcontribution of the desired lowering h_(b−TGT) of the centre B ofgravity) depending on the transverse acceleration a_(y) by means of alinear law L2. The linear law L2 is symmetrical, in a mirror-likemanner, for positive and negative transverse accelerations a_(y)(symmetrical in a mirror-like manner, as the desired lowering h_(b−TGT)of the centre B of gravity always is negative regardless of the sign ofthe transverse acceleration a_(y)) and entails, in any operatingcondition, proportionally increasing the desired lowering h_(b−TGT) ofthe centre B of gravity as the absolute value of the transverseacceleration a_(y) increases; in particular, the linear law L2 entails azero desired lowering h_(b−TGT) of the centre B of gravity, only whenthe absolute value of the transverse acceleration a_(y) is zero, and itentails a linear variation of the desired lowering h_(b−TGT) of thecentre B of gravity from a zero value, when the absolute value of thetransverse acceleration a_(y) is zero, to a maximum value VMAX2, whenthe absolute value of the transverse acceleration a_(y) is maximum.

In the embodiment shown in FIG. 6 , the value VMAX2 is equal (inabsolute value) to 22.5 mm (in FIG. 6 , the value VMAX2 is negative toindicate that it is a lowering) and is reached when the transverseacceleration a_(y) is equal (in absolute value) to 1.5 g.

As mentioned above, the calculation block 18 uses a mathematical modelof the road vehicle 1, which determines a target force F_(1 . . . 4−TGT)for the actuator 10 of each active shock absorber 6 (also) depending onthe desired lowering h_(b−TGT) of the centre B of gravity; as aconsequence, the actuator 10 of each shock absorber is controlled so asto pursue the corresponding target force F_(1 . . . 4−TGT).

According to a preferred embodiment, the desired lowering h_(b−TGT) ofthe centre B of gravity is established depending both on thelongitudinal acceleration a_(x) and on the transverse accelerationa_(y); according to a different embodiment, the desired loweringh_(b−TGT) of the centre B of gravity is established depending on thesole longitudinal acceleration a_(x) or on the sole transverseacceleration a_(y).

According to FIG. 7 , the control bock 17 establishes the desired rollangle φ_(TGT) depending on the sole transverse acceleration a_(y) bymeans of a linear law L3. The linear law L3 is symmetrical for positiveand negative transverse accelerations a_(y) (namely, the roll angleφ_(PTGT) always is positive, when the transverse acceleration a_(y) ispositive, and the roll angle φ_(TGT) always is negative, when thetransverse acceleration a_(y) is negative) and entails linearly varyingthe desired roll angle φ_(TGT) by a value ranging from 1.0° to 1.8° (andpreferably equal to 1.4°) for each 1 g increase in the absolute value ofthe transverse acceleration a_(y); namely, the linear law L3 entails agradient (variation of the angle per unit of acceleration) ranging from1.0°/g to 1.8° /g and preferably equal to 1.4°/g. As a consequence, thelinear law L3 entails proportionally varying the absolute value of thedesired roll angle φ_(PTGT) as the absolute value of the transverseacceleration a_(y) increases and, in particular, the linear law L3entails a zero desired roll angle φ_(TGT) only when the absolute valueof the transverse acceleration a_(y) is zero; in other words, the linearlaw L3 entails a linear variation of the desired roll angle φ_(TGT) froma zero value, when the absolute value of the transverse accelerationa_(y) is zero, to a maximum absolute value VMAX3, when the absolutevalue of the transverse acceleration a_(y) is maximum.

In the embodiment shown in FIG. 7 , the value VMAX3 ranges, in absolutevalue, from 1.8° to 2.4° and preferably is equal to 2.1° and is reachedwhen the transverse acceleration a_(y) is equal (in absolute value) to1.5 g.

According to FIG. 8 , the control bock 17 establishes the desired pitchangle θ_(TGT) depending on the sole longitudinal acceleration a_(x) bymeans of a linear law L4. The linear law L4 is not symmetrical forpositive or negative transverse accelerations a_(y) (namely, the linearlaw L4 is unbalanced towards positive pitch angles θ_(TGT), which entaillowering the front part of the road vehicle 1 and lifting the rear partof the road vehicle 1) and entails varying the desired pitch angleθ_(TGT) by a value ranging from 1.2° to 2.0° (preferably equal to 1.6°)for each 1 g increase in the absolute value of the longitudinalacceleration a_(x); namely, the linear law L4 entails a gradient(variation of the angle per unit of acceleration) ranging from 1.2°/g to2.0°/g and preferably equal to 1.6°/g.

The linear law L4 entails proportionally increasing the desired pitchangle θ_(TGT) as the value of the longitudinal acceleration a_(x)decreases. In particular, the linear law L4 entails a positive pitchangle θ_(TGT) greater than zero, when the value of the longitudinalacceleration a_(x) is zero, and entails a negative pitch angle θ_(TGT),when the value of the longitudinal acceleration a_(x) is positive (roadvehicle 1 accelerating) and preferably greater than at least 0.3 g; inparticular, the linear law L4 entails a zero pitch angle θ_(TGT) whenthe value of the longitudinal acceleration a_(x) is positive (namely,the road vehicle 1 is accelerating) and ranges from 0.3 g to 0.5 g(preferably is equal to 0.4 g).

In the embodiment shown in FIG. 8 , the linear law L4 entails that thedesired pitch angle θ_(TGT) ranges from +2.0° to −0.5° in case ofmaximum deceleration and maximum acceleration, respectively.

As mentioned above, the calculation block 18 uses a mathematical modelof the road vehicle 1, which determines a target force F_(1 . . . 4−TGT)for the actuator 10 of each active shock absorber 6 (also) depending onthe desired roll angle φ_(TGT) and on the desired pitch angle θ_(TGT);as a consequence, the actuator 10 of each shock absorber is controlledso as to pursue the corresponding target force F_(1 . . . 4−TGT).

According to a preferred embodiment, the calculation block 18 (whichestablishes a desired roll angle φ_(TGT) and determines a target forceF_(1 . . . 4−TGT) for the actuator 10 of each active shock absorber 6depending on the desired roll angle φ_(TGT)) determines a totalanti-roll moment depending on the desired roll angle φ_(TGT) (namely, atotal anti-roll moment which allows the desired roll angle φ_(TGT) to beobtained), establishes a distribution of the total anti-roll momentbetween a front axle (comprising the two front wheels 2) and a rear axle(comprising the two rear wheels 2) and determines the target forceF_(1 . . . 4−TGT) for the actuator 10 of each active shock absorber 6depending on the total anti-roll moment and also depending on thedistribution of the total anti-roll moment between the front axle andthe rear axle.

The total anti-roll moment is traditionally distributed in a symmetricalmanner between the front axle and the rear axle, namely an anti-rollmoment generated for the front axle always is equal to an anti-rollmoment generated for the rear axle. It has been observed that anasymmetrical distribution of the total anti-roll moment is advantageous,namely the anti-roll moment generated for the front axle is differentfrom the anti-roll moment generated for the rear axle; furthermore, thedistribution of the total anti-roll moment can be changed (by movingpart of the total anti-roll moment from the front axle to the rear axleor vice versa) depending on the moving state of the road vehicle 1.

According to a preferred embodiment, the distribution of the totalanti-roll moment entails a lower limit value, which determines anincrease in the anti-roll moment of the rear axle and ranges from −12%to −6% (namely, the anti-roll moment of the rear axle is greater thanthe anti-roll moment of the front axle by 12%-6%), and an upper limitvalue, which determines an increase in the anti-roll moment of the frontaxle and ranges from +1.5% to +4% (namely, the anti-roll moment of thefront axle is greater than the anti-roll moment of the rear axle by1.5%-4%).

The calculation block 18 determines when the road vehicle 1 is about tostart a bend or is in the middle of a bend and establishes adistribution of the total anti-roll moment more unbalanced towards therear axle, when the road vehicle 1 is about to start a bend or is in themiddle of a bend. Furthermore, the calculation block 18 determines whenthe road vehicle 1 is exiting a bend and establishes a distribution ofthe total anti-roll moment less unbalanced towards the front axle, whenthe road vehicle 1 is exiting a bend.

The calculation block 18 determines whether the road vehicle 1 has anoversteering behaviour and unbalances the distribution of the totalanti-roll moment towards the front axle, when the road vehicle 1 has anoversteering behaviour (so as to counter the oversteering behaviour inorder to try and give the road vehicle 1 more neutral behaviour).Similarly, the calculation block 18 determines whether the road vehicle1 has an understeering behaviour and unbalances the distribution of thetotal anti-roll moment towards the rear axle, when the road vehicle 1has an understeering behaviour (so as to counter the understeeringbehaviour in order to try and give the road vehicle 1 more neutralbehaviour).

The total anti-roll moment introduced by the roll control canarbitrarily be distributed between the front axle and the rear axle; thechoice of this distribution affects the distribution of the transfer oflateral load between the two axles, keeping the total transferunchanged. In general, a distribution more unbalanced towards the rearaxle increases the maximum lateral acceleration, for it delays thesaturation of the front axle; therefore, this configuration is to bepreferred when the vehicle is about to start or is in the middle of abend with pure lateral slip. A distribution shifted to the front axle,on the other hand, decreases the maximum lateral acceleration, advancingthe saturation of the front axle, but, at the same time, it privilegesthe rear axle, making it capable of transferring more longitudinalforce; therefore, this configuration is to be preferred during thetraction phase exiting a curve. The distribution of the anti-roll momentcan dynamically be changed in order to put the road vehicle 1 in abetter condition in the different phases of the bend; by way of example,the distribution of the anti-roll moment could entail a value of −8.5%(when the vehicle is about to start or is in the middle of the bend) and−3% (exiting the bend), thus using a variation of approximately 5%.

The combination of control of the height of the centre B of gravity andcontrol of the roll angle φ allows the angle on the inside of the bendto be maintained still and the position of the angle on the outside ofthe bend to be left unchanged; an ideal behaviour is obtained byimposing a desired roll angle (PTGT corresponding to half the roll angleφ of the non-controlled road vehicle 1 (namely, with the active shockabsorbers 6 turned off and, hence, only in passive mode) and by imposinga desired lowering h_(b−TGT) of the centre B of gravity equal to halfthe lowering of the angle on the outside of the bend of thenon-controlled road vehicle 1 (namely, with the active shock absorbers 6turned off and, hence, only in passive mode). This functionality dependson the transverse acceleration a_(y), specifically, when driving along abend, a downward force is activated in the angles on the inside of thebend in order to avoid the extension thereof so as to lower the centre Bof gravity and reduce the roll; in order to further reduce the rollgradient, an action upon the outer angle is also needed through anupward force (which, anyway, is much smaller than the force required forthe inner angle).

The combination of the controls of the height of the centre B of gravityand of the static pitch angle θ leads to a lowering of the centre B ofgravity through differentiation of the heights from the ground betweenfront and rear axle, so as to work in the best point of aerodynamicefficiency. This functionality is important both for absoluteperformances, in fact the vertical load can be increased with a properpositioning according to a predetermined aerodynamic map, and forconsumptions, since the resistance to the forward movement of thevehicle can be reduced when driving along a straight road. For example,the control could not entail any height unbalance between the two axlesfor speeds below 100 km/h, an increase in the vertical load for speedsexceeding 100 km/h through a greater lowering of the front axle comparedto the rear axle and a lowering of the front axle also along a straightroad in order to decrease the resistance to the forward movement.

The combination of the controls of the height of the centre B of gravityand of the pitch angle θ also optimizes stopping spaces. Thanks to thelowering of the centre of gravity, there is a smaller transfer oflongitudinal load with a consequent increase in the overall grip; on theother hand, thanks to the control of both the static and the dynamicpitch angle θ, there is a quicker settling of vertical and longitudinalforces.

The embodiments described herein can be combined with one another,without for this reason going beyond the scope of protection of theinvention.

The control method described above has different advantages.

First of all, the control method described above increases theperformances of the road vehicle 1 when driving close to the grip limit(typically on a track) both along bends and along straight roads (duringthe acceleration when exiting a bend or during the deceleration whenentering a bend).

In particular, the control method described above reduces both staticand dynamic roll as well as both static and dynamic pitch and thesereductions can directly be perceived by the driver, who, then, gets theimpression of a road vehicle 1 that is more stable and, hence, more fun(safer) to be driven. Furthermore, the lowering of the centre B ofgravity determines an increase in performances, for it reduces thetransfer of absolute load, and, hence, allows all wheels 2 to work attheir limit. The reduction in static and dynamic pitch improves brakingperformances. The dynamic distribution of the anti-roll moment improvesbraking performances, since it leads to an ideal management of thefriction ellipse of the tyres of the wheels 2.

Namely, the control method described above improves pure performances,reducing lap times, and also improves the driving fun, giving driversthe feeling of a more usable car.

Furthermore, the control method described above is particularly sturdyand safe in all conditions, namely it has a basically zero risk ofcontrol errors that can generate anomalous oscillations orover-elongations of the suspensions 5.

Finally, the control method described above is simple and economic to beimplemented, for it does not require either a significant calculationability or a large memory space.

LIST OF THE REFERENCE NUMBERS OF THE FIGURES

1 vehicle

2 wheels

3 hub

4 frame

5 suspension

6 active shock absorber

7 element

8 element

9 spring

10 electric actuator

11 position sensor

12 control unit

13 longitudinal accelerometer

14 transverse accelerometer

15 estimating block

16 estimating block

17 control block

18 control block

ax longitudinal acceleration

ay transverse acceleration

B centre of gravity

α steering angle

β attitude angle

h_(b−TGT) desired lowering of the centre of gravity

F force

F_(TGT) desired force

φ roll angle

θ pitch angle

ψ yaw angle

α steering angle

Vψ yaw speed

L1 law

L2 law

L3 law

L4 law

1. A method to control active shock absorbers (6) of a road vehicle (1);each active shock absorber (6) is part of a suspension (5) connecting aframe (4) to a hub (3) of a wheel (2) and is provided with an actuator(10); the control method comprises the steps of: determining alongitudinal acceleration (a_(x)) and a transverse acceleration (a_(y))of the road vehicle (1); establishing a first contribution of a desiredlowering (h_(b−TGT)) of a centre (B) of gravity of the road vehicle (1)only depending on the longitudinal acceleration (a_(x)); establishing asecond contribution of the desired lowering (h_(b−TGT)) of the centre(B) of gravity of the road vehicle (1) only depending on the transverseacceleration (a_(y)); establishing a desired lowering (h_(b−TGT)) of thecentre (B) of gravity of the road vehicle (1) that is equal to thecontribution having the greater absolute value; and controlling theactuator (10) of each active shock absorber (6) so as to obtain thedesired lowering (h_(b−TGT)) of the centre (B) of gravity.
 2. Thecontrol method according to claim 1, wherein the first contribution isestablished depending on the longitudinal acceleration (a_(x)) by meansof a first linear law (L1).
 3. The control method according to claim 2,wherein the first linear law (L1) entails proportionally increasing thedesired lowering (h_(b−TGT)) of the centre (B) of gravity as theabsolute value of the longitudinal acceleration (a_(x)) increases. 4.The control method according to claim 2, wherein the first linear law(L1) entails a zero desired lowering (h_(b−TGT)) of the centre (B) ofgravity when the absolute value of the longitudinal acceleration (a_(x))is smaller than a first threshold value (TH1).
 5. The control methodaccording to claim 2, wherein the first linear law (L1) entails aconstant desired lowering(h_(b−TGT) of the centre (B) of gravity, which is equal to a maximum value (VMAX)1),when the absolute value of the longitudinal acceleration (a_(x)) isgreater than a second threshold value (TH2).
 6. The control methodaccording to claim 2, wherein: the first linear law (L1) entails a zerodesired lowering (h_(b−TGT)) of the centre (B) of gravity when theabsolute value of the longitudinal acceleration (a_(x)) is smaller thana first threshold value (TH1); the first linear law (L1) entails aconstant desired lowering (h_(b−TGT)) of the centre (B) of gravity,which is equal to a maximum value (VMAX1), when the absolute value ofthe longitudinal acceleration (a_(x)) is greater than a second thresholdvalue (TH2); and the first linear law (L1) entails a linear variation ofthe desired lowering (h_(b−TGT)) of the centre (B) of gravity from thezero value, when the absolute value of the longitudinal acceleration(a_(x)) is equal to the first threshold value (TH1), to the maximumvalue (VMAX1), when the absolute value of the longitudinal acceleration(a_(x)) is equal to the second threshold value (TH2).
 7. The controlmethod according to claim 1, wherein the second contribution isestablished depending on the transverse acceleration (a_(y)) by means ofa second linear law (L2).
 8. The control method according to claim 7,wherein the second linear law (L2) entails proportionally increasing thedesired lowering (h_(b−TGT)) of the centre (B) of gravity as theabsolute value of the transverse acceleration (a_(y)) increases.
 9. Thecontrol method according to claim 7, wherein the second linear law (L2)entails a zero desired lowering (h_(b−TGT)) of the centre (B) of gravityonly when the absolute value of the transverse acceleration (a_(y)) iszero.
 10. The control method according to claim 7, wherein the secondlinear law (L2) entails a linear variation of the desired lowering(h_(b−TGT)) of the centre (B) of gravity from a zero value when theabsolute value of the transverse acceleration (a_(y)) is zero to amaximum value (VMAX2) when the absolute value of the transverseacceleration (a_(y)) is maximum.
 11. The control method according toclaim 1 and comprising the further steps of: using a mathematical modelof the road vehicle (1) to determine a target force (F_(1 . . . 4−TGT))for the actuator (10) of each active shock absorber (6) depending on thedesired lowering (h_(b−TGT)) of the centre (B) of gravity; andcontrolling the actuator (10) of each shock absorber so as to pursue thecorresponding target force (F_(1 . . . 4−TGT)).
 12. The control methodaccording to claim 1, wherein the desired lowering (h_(b−TGT)) of thecentre (B) of gravity is recalculated with a frequency ranging from 1 to4 Hz.